Corrections

The typing rule FunTyp (Figure 3) now has "x:tau1" in the premise
instead of the incorrect "x:tau2".

The typing rules now use abstract syntax consistently, instead of sometimes
using concrete syntax.

The Typing section claimed that a function "typing" was declared
in the starting code file typing.sml; there is no such declaration.
The new handout asks you to write the function, instead of asking you
to complete it.